Police search the Pembrokeshire coastal path at Little Haven after discovering the murdered couple Peter and Gwenda Dixon in June 1989. Photograph: Rex Features

The brutal double murder of husband and wife Peter and Gwenda Dixon as they walked on the Pembrokeshire coastal path in the summer of 1989 was a particularly shocking crime.

The holidaymakers were blasted to death at close quarters with a shotgun and their bodies hidden close to a sheer cliff concealed by a screen of branches interwoven with ferns and other foliage. Mrs Dixon appeared to have been sexually assaulted, and Mr Dixon had been forced to reveal his bankcard pin.

A record number of viewers contacted the BBC's Crimewatch, while detectives took almost 4,000 statements. It became a real-life murder mystery as officers investigated reports of strange divers in the bay, magnified a picture of a cow found on the Dixons' camera so they could read the number on its ear tag and thus trace its owner, and travelled far and wide to speak to people who had signed the visitors' book in a nearby church.

Most intriguingly, detectives looked at possible links between the murders and the IRA, after an arms dump was discovered close to where the bodies were found. Could the Dixons have been murdered after happening upon members of the IRA as they smuggled arms into west Wales?

Now, more than 20 years on, after a cold case review, former farm labourer and welder's mate John Cooper has been convicted of murdering the Dixons and another double killing, that of siblings Richard and Helen Thomas at their remote farmhouse four years earlier. Cooper was also found guilty of raping a 16-year-old girl and sexually assaulting her 15-year-old friend.

Police believe that Cooper, already a convicted armed robber and burglar, was motivated by greed and sexual gratification and was able to carry out the attacks – and remain undetected for so long – because of his intimate knowledge of the area and his skills as an outdoorsman.

Cooper first struck just before Christmas 1985. Richard and Helen Thomas, aged 58 and 54, were found in the burnt-out ruins of their farmhouse, near Milford Haven. They had been blasted by a shotgun. At the peak of the investigation a year later, 150 officers were involved.

That double murder remained unsolved when, in the summer of 1989, Tim Dixon reported that his parents, Peter and Gwenda, aged 51 and 52, had failed to return to their Oxfordshire home after a camping trip to Pembrokeshire.

Their bodies were found concealed close to the coastal path on the edge of a 60-metre-high cliff, about six miles from the spot where the Thomases had been shot dead. Walkers had spotted swarms of flies and told the police.

Mr Dixon's hands had been tied behind his back. His wife's body was naked from the waist down. Police believe she had been sexually assaulted. After their deaths, Mr Dixon's cash card was used to withdraw money, indicating that he had been forced to tell the attacker his pin. Again there was a huge police hunt; again nobody was caught.

Then in March 1996, at dusk in a field near a housing estate in Milford Haven, a man wearing a balaclava and armed with a sawn-off double-barrelled shotgun approached five teenagers. He dragged away one member of the group, a 16-year-old girl, and raped her at knifepoint before indecently assaulting a 15-year-old girl. As he left, the man fired the shotgun into the air.

Two years later, in December 1998, John Cooper was jailed for 16 years for armed robbery and 30 burglaries.

Police found that he used hedgerows around his home near Milford Haven as "safes" to hide stolen valuables, tools and weapons, among them a Belgian-made shotgun. He would carefully prepare escape routes by cutting holes in fences and hedges. He was proud of his survival skills had a manual that he claimed was an "SAS handbook".

Police could not link him to the two double murders or the sexual assaults. Nevertheless, he was jailed for 16 years.

The failure to track down the killers of the Dixons and the Thomases continued to rankle. In 2006, Dyfed-Powys police formed Operation Ottawa to look again at all the evidence. Two years later they quizzed Cooper, who was still in prison, for four days. He denied all involvement.

He was freed on licence at the end of 2008 and the breakthrough finally came in April and May 2009 when new tests indicated a link between Cooper and the notorious crimes.

Scientists had re-examined 600 exhibits, but among the most telling was the Belgian-made shotgun. Underneath a coat of black paint on the weapon they found a spot of blood that matched Peter Dixon's.

A pair of shorts found at Cooper's home when he was arrested over the burglaries also proved damning. On the left leg was a blood mark that tests showed must have been Peter Dixon's. Inside a hem was a trace of human fluid that matched the DNA profile of the Dixons' daughter, Julie.

Police believe that when Cooper killed the Dixons, his own trousers were spattered with blood. They believe he must have gone home in Mr Dixon's shorts and continued to use them as his own.

Cooper was charged in May 2009, 20 years after the Dixons were murdered, and 24 after the Thomases were shot dead. His conviction provides another example of how improving technology can be used to solve historic cases.

The IRA had nothing to do with the Dixons' deaths. The mysterious divers in the bay were irrelevant. As the jury was told in court, both the Dixons and the Thomases were the victims of "merciless executions" for "pitifully small financial gain".